The invention relates to a communication system, a method for operating a communication system, a server unit, a method for operating a server unit, a communication service client unit and a method for operating a communication service client unit.
The communication service Push-to-talk-over-Cellular (PoC) allows a user of a mobile radio participant terminal to transmit voice data to one or more receivers simultaneously.
For this, there is typically a special PoC key on the mobile radio participant terminal which, when operated, allows the user to start to input voice data.
The voice data are usually distributed, that is to say transmitted to the desired receiver(s), using a mobile radio communication network while they are actually being input. This process is called “streaming”.
Transmission takes place using the half-duplex method, that is to say that during the voice input and during the transmission, only the sender, that is to say the user who is inputting and sending the voice data, can transmit voice data to the receivers, but the receivers cannot simultaneously send voice data to the sender. In particular, the sender cannot be interrupted by the receivers.
Clearly, communicating using PoC is equivalent to conventional CB radio from the user's standpoint, but with the extension that the sender can transmit voice data worldwide to receivers, who can be reached using the suitable switching technology of at least one mobile radio communication network.
If a user of PoC wishes to send voice messages to the same receiver relatively often, PoC allows him to define personal, fixed user groups. By way of example, a user of PoC can define a group labelled “friends”, containing relevant members and their respective address, for example an SIP-URL (Session Initiation Protocol Uniform Resource Locator) in the form of a telephone number or in the form of an SIP address.
This group can then be assigned its own group address in the form of an SIP-URL, and when a PoC session is set up, that is to say a communication session using PoC, by indicating the group address initiated by a user all members of the group are addressed by a PoC server computer and are invited to join the PoC session.
The prerequisite for a member of the group being able to be invited is that the member is registered, that is to say “online”, in the mobile radio communication network which is being used to provide the PoC used.
Users of PoC who are involved in a PoC session actively, that is to say as senders, or passively, that is to say as receivers, are subsequently called PoC participants in the PoC session.
Group management, as described in 3GPP TS 22.250 V6.0.0 (2002-12), “IP Multimedia Subsystem (IMS) group management” and Push to Talk over Cellular (PoC); List Management and Do-not-Disturb; PoC Release 2.0, allows simple handling of groups within the context of PoC. Alternatively, groups can be used within the context of other communication services. By way of example, a user can use an appropriate group to send an MMS (Multimedia Message Service) message to all members of his family.
In the case of PoC, a user can use an appropriate group to start a PoC session with all the members of his skat club, for example. To this end, a PoC communication network, i.e. a communication network providing PoC, contains a group management server (GM server) which the user can use to create and manage a group. The user is called the administrator of the group.
In line with the prior art, the main components of the specification of a group are as follows:                Group identifier: this is used to provide the group with unique identification. By way of example, its form is sip:myfriends@myname.t-mobile.de        Group specific attributes: these attributes specify more precise properties for the group. These are:                    Group information: information in the form of a simple text (for example “This is my family”)            Group visibility: this specifies which users can find the group (for example using a search function on the GM server). By way of example, the group visibility specifies that only the administrator of the group can find the group.            Group duration: this specifies for how long and/or when the group is valid or can be used. By way of example, group duration may specify that the group of “football stadium friends” for a user can be used only on Saturdays between 2 o'clock and 6 o'clock pm.            Service specific info: this is information specific to the communication service within whose context the group can be used. By way of example, within the context of PoC, there is a distinction between “pre-arranged groups” and “chat groups”. Thus, if the group is to be used within the context of PoC, the service specific info can be used to indicate what type of group is involved.                        Group members: this is a list of users for such groups belonging to the group, that is to say of group members. Each group member, which may itself be a group, in particular, is clearly specified by means of an ID (identifier, for example an SIP URI). In addition, the following attributes may be stipulated for each group member:                    Member rights: these specify the rights of the group member            Anonymity: this specifies whether or not the group member is anonymous during communication within the group            Service specific info: this is information specific to the communication service. In the case of PoC, for example, the function of a moderator of a PoC session can be allocated to a group member using the service specific info.                        
A user with the relevant right, for example the administrator of a group, can in line with the prior art, perform the following group management operations as part of group management for the group:                Manipulation of groups                    Get a list of groups            Create a new group            Delete a group            Modify group attributes                        Manipulation of members in a group                    Get a list of members            Add a member to a group            Delete a member from a group            Modify member attributes                        
Within the context of PoC, a group is used by a user in the following manner, for example, as explained with reference to FIG. 1.
FIG. 1 shows a message flow diagram 100 based on the prior art.
In step 106, the user, taken as the user of a first PoC client unit 101, creates a group (PoC group), to which a second PoC client unit 102 (or the relevant user) and a third PoC client unit 103 (or the relevant user) belong, in a GM server computer 104 by sending a first message 120. By way of example, the PoC group is allocated the ID (identifier) sip:myfriends@abc.de, and the user is notified of this by means of a second message 121, which is sent to the first PoC client unit 101 by the GM server computer 104 in step 107.
In step 108, the user selects the PoC group. In step 109, the user starts a PoC session with the PoC group. To this end, he uses the first PoC client unit 101 to send a third message 122 to a PoC server computer 105. In step 110, the PoC server computer 105 establishes that the ID specified in the third message 122 (sip:myfriends@abc.de) specifies a PoC group. The PoC server computer 105 then sends a fourth message 123 to the GM server computer 104 in step 111 in order to resolve this PoC group, i.e. in order to ascertain which group members are part of this PoC group. The GM server computer 104 then uses a fifth message 124 in step 112 to send a list of all the group members in the PoC group to the PoC server computer. In this example, the group contains the second PoC client unit 102 and the third PoC client unit 103.
By sending a sixth message 125 in step 113 to the second PoC client unit 102 and by sending a seventh message 126 to the third PoC client unit 103, the PoC server computer 105 invites all members of the PoC group to join the PoC session which is to be set up. As soon as the first group member accepts the invitation in step 114 using an eighth message 127, in this example the second PoC client unit 102, a ninth message 128 is sent in step 116 to the initiator of the PoC session, i.e. to the first PoC client unit 101, signalling that the PoC session has now started and that voice packets can be sent within the context of the PoC session.
In line with the prior art, when a group is defined, for example when a group is created in a GM server, the members of the group need to be listed. Particularly the stipulation of which members the group contains is very static. In the case of a group which contains all the members of a user's family as group members, this is not a serious drawback, since the members of a user's family do not change very often.
In the case of a taxi operator, for example, wishing to create a user group whose group members are all his associated taxis (or the relevant drivers) which are currently free, it is very inconvenient to perform the group management operation “Add a member to the group” or “Delete a member from the group” on the GM server computer as soon as a taxi becomes free or comes into service.
Besides the considerable complexity for the taxi operator and a resultant low level of user friendliness, this leads to a very high volume of signalling traffic for the messages to the GM server, for example on the air interface of a mobile radio communication system used for communication.
In addition, the information for deciding who is currently in turn to be a member of a group may not be available to the user (for example in his radio mobile participant terminal). The user may need to go to considerable lengths to ascertain this information.
In the case of a taxi operator, the taxi operator (or his mobile radio participant terminal, for example) needs to be notified every time a taxi becomes free or comes into service, so that the taxi operator always has the current level of information. Constant transmission of notification messages likewise results in a very high volume of signalling traffic, for example on the air interface of the mobile radio communication system used for communication.
Group management operations using HTTP are described in Push to Talk over Cellular (PoC); List Management and Do-not-Disturb; PoC Release 2.0. HTTP get instructions are described in RFC “Hypertext Transfer Protocol—HTTP/1.1”.
RF3261 “SIP: Session Initiation Protocol” describes SIP INVITE, RFC3265 “Session Initiation Protocol (SIP)-Specific Event Notification” describes SIP SUBSCRIBE and RFC3428 “Session Initiation Protocol (SIP) Extension for Instant Messaging”, describes SIP MESSAGE. These are methods based on the SIP (Session Initiation Protocol).
WO 00/16209 describes a method for exchanging e-mails in which a user can register with a server and can indicate criteria specifying those other users to which e-mails he has sent are to be sent and can indicate a profile which is used to decide whether e-mails sent by other users are sent to him.
WO 02/103570 A1 discloses a network-based system and a method for dynamically managing user groups. Periodically dynamic user data are compared with group membership criteria in order to determine the user groups.
US 2002/0107008 A1 discloses a communication system in which a communication terminal selects the participants in a communication session from a list of possible participants in the communication session on the basis of a geographical distance criterion.
US 2004/0203907 A1 discloses a communication system in which the participants in a communication session are selected from a group of possible participants in the communication session on the basis of the geographical locations at which the possible participants are respectively located.